1. Field of the Invention
The present invention relates to a filament supporting structure of a fluorescent display tube having an anode plate with phosphor segments and filament cathodes inside an airtight container.
2. Description of Related Art
A reduction in power consumption of a fluorescent display tube has been demanded. To that end, filament cathodes are divided into blocks and are driven separately for each block so that current is not carried to the filament cathodes corresponding to a non-displaying region.
A fluorescent display tube, shown in FIG. 7, is a light emitting device including an airtight container which is provided therein with an anode substrate having phosphor segments 130, 131 made of electron-excited phosphors formed on an insulating substrate, a front plate and side plates, all of which are sealed in the airtight container by low-melting-point glass, and inside the container is provided with filament cathodes 430, 431 which emit thermal electrons so the phosphors emit light when receiving the thermal electrons.
Another example of the fluorescent display tube includes a grid electrode between the phosphors 130, 131 and the filaments 430, 431, and performs acceleration control of the thermal electrons emitted from the filaments 430, 431.
Japanese Utility Model Publication Number H02-5872 discloses a fluorescent display tube having the filaments 430, 431 divided into several blocks and extended across the container, so a voltage can be applied separately to filaments for each block.
The filament cathodes extending within the airtight container are tungsten wires having a diameter of for example about 14.6 μm coated with electron emitting substance layer composed of ternary oxides (Ba, Sr, Ca)O.
During the use, the filament cathodes are heated to about 600 to 650° C. Therefore, in order to electrically heat the tungsten wires having diameter of about 14.6 μm, voltage of 3 to 6V is applied so current of about 25 mA flows through each filament cathode (refer for example to non-patent literature, “Fluorescent Display Tube, published by Sangyo-Tosho, October 1990, p. 19-21”).
In conventional art, for example Japanese Patent Publication Number H09-147766 discloses providing two cathode-supporting plates on each end of a rectangular anode substrate. The cathode-supporting plate is an elongated plate and is arranged so as a longitudinal direction thereof is perpendicular to a longitudinal direction of the filament cathode (a longitudinal direction of the anode substrate). The cathode-supporting plates are arranged in the longitudinal direction of the anode substrate with a space. The cathode-supporting member is fixed to a supporting plate composed of 426 alloy (composed of 42% of Ni, 6% of Cr and the rest is Fe alloy) with a thermal expansion coefficient close to that of glass. Each filament cathode is tensioned by one end thereof attached to the cathode-supporting member and the other end thereof attached to an anchor member fixed to the supporting plate. The supporting plate can be guided outside the container where they are connected to a cathode driving circuit.
Furthermore, Japanese Utility Model Publication Number S63-18750 discloses a fluorescent display tube having heat-producing wires. Each heat-producing wire of an adjacent indirectly-heating-type cathodes has one end attached to the same metal supporting plate and each other end thereof is attached, respectively, to different and separate metal supporting plates connected to the wires of other adjacent cathodes. The wire ends of the outermost cathode not connected to the common metal supporting plates are attached to the metal supporting plates connected to external lead wires for electrical connection.
Since the filament cathode is structured by the tungsten wire, the magnitude of applied voltage is proportional to a length of the filament cathode. According to the conventional art, reduction in the size of the fluorescent display tube leads to shortening of the length of the filament cathodes. Therefore, while the voltage employed in a filament power source for a typical fluorescent display tube is about 3V, the voltage applied to the fluorescent display tube with short filament length is for example only about 2V, causing it to be difficult to handle.
In view of the above-described problem, an object of the invention is to provide a fluorescent display tube and a driving method thereof enabling a use of a filament power source with voltage of 3 to 8V and reducing current which flows through the filament cathode arranged to extend within the fluorescent display tube.